As containers for contents such as various foods or pet foods, cup-shaped or tray-shaped containers each comprising suitable resin and having a shape including a bottom wall, a side wall extending upward from the bottom wall, and a flange section extending radially outwardly from the upper end of the side wall are in wide practical use. In using such a container, it is common practice to fill contents into the container, then place a flange of a molded lid comprising a plastic synthetic resin on the flange section of the container, and then secure the flange of the lid to the flange section of the container for sealing. In connection with the sealing of such a type of container, Patent Document 1 to be described below discloses a device for producing a sealed container, which comprises a container transport means for transporting a container, and a rotary disk body rotating at the same speed as the transport speed of the container transport means, and includes a lid fixing means for pressing a lid mounted on the trunk of the container, wherein a plurality of the lid fixing means are arranged with equal spacing at the peripheral edge of the rotary disk body, and locations of the trunk of the container and the lid to be scheduled for welding are irradiated with laser light.
Patent Document 2 to be described below, on the other hand, discloses a sealing method by laser welding of a container and a lid, wherein one of the container and the lid comprises a thermoplastic resin permeable to laser light, while the other of them comprises a resin composition composed of a thermoplastic resin containing an exothermic substance which absorbs laser light to generate heat, a laser for the laser light is a laser capable of throwing a rectangular beam uniform in intensity distribution, and the range of the upper limit of a temperature to be raised by laser irradiation, and the conditions for reaching the upper limit temperature are defined for the welding interface between the lid and the container.